1. Field of the Invention
The present invention relates to an image pick-up unit which includes an image pick-up device and optical filter layers positioned in front of the image pick-up device. The image pick-up unit is suitable for an electronic camera having a focal plane shutter such as electronic SLR camera.
2. Description of the Related Art
In electronic apparatuses using a solid-state image pick-up device (e.g., CCD or CMOS) such as digital still cameras or digital video cameras, it is generally the case that a cemented optical element, consisting of at least one absorptive type of infrared (IR) cut-off filter and at least one optical low-pass filter which are cemented to each other, is disposed between an image pick-up lens (e.g., photographing lens) and the solid-state image pick-up device.
FIG. 9 shows an arrangement of a conventional image pick-up device and associated optical elements. In a conventional image pick-up device 101 (CCD or CMOS package), a cover glass 103 is cemented to a front surface of a package of the image pick-up device 101 to cover the front of the image pick-up device 101 in an airtight fashion. A multilayer optical filter F having a first optical low-pass filter 106, an infrared cut-off filter 105 and a second optical low-pass filter 104, which are cemented together and arranged in that order from the object side (left side as viewed in FIG. 9), is positioned in front of the cover glass 103. The first optical low-pass filter 106 is cemented to the front of the infrared cut-off filter 105 while the second optical low-pass filter 104 is cemented to the rear of the infrared cut-off filter 105. The multilayer optical filter F is positioned in front of the cover glass 103 with a space 110 between the second optical low-pass filter 104 and the cover glass 103.
In the case where the image pick-up device 101 and the multilayer optical filter F are incorporated in a camera body of a conventional SLR camera which uses a silver-salt film, it is difficult to design each of the first optical low-pass filter 106, the infrared cut-off filter 105 and the second optical low-pass filter 104 because the distance between the focal plane shutter and the image pick-up device 101 is narrow.
To this end, the inventors of the present invention have proposed a Japanese Unexamined Patent Publication No. 2001-284561 in which optical filters corresponding to the aforementioned optical filters 104, 105 and 106 are cemented together to be formed as a multilayer optical filter which is cemented to a cover glass corresponding to the aforementioned cover glass 103. In this case, if each of the optical filters corresponding to the aforementioned optical filters 104, 105 and 106 is formed as thin as possible, the strength of each optical filter becomes low, while it becomes difficult to distinguish between the front side and the rear side of each optical filter. Some expertise is also required to cement the optical filters together.